High-intensity operation of geiger tubes



March 16, 1954 s. w. LICHTMAN 2,672,561

HIGH-INTENSITY OPERATION OF GEIIGER TUBES Filed Dec. 5, 1950 2 Sheets-Sheet 1 l3 RADIO ACTIVITY DETECTOR F COUNT T INDICATOR '2 CIRCuIT l5 PULSE 1 25 GENERATOR l6 \l. o- T- ZELELE RADIO ACTIVITY DETECTOR F BI T |;%)TRE ZL N ICA Q CIRCuIT 1 I5 c PULSE 25 l7 GENERATOR U PULSE GENERATOR Io UNTIL/' .IL Ll I W W EE 23 I 2| I i 17 TO COuNT RATE I INDICATOR cIRCuIT I I6 I INVENTOR.

SAMUEL W. LICHTMAN ATTORN KY3 Patented Mar. 16, I954 HIGH-INTENSITY OPERATION OF GEIGER TUBES Samuel W. Lichtman, Oxon Hill, Md. Application December 1950, Serial No. 199,274

4 Claims.

(Granted under Title 35, U. S. Code (1952),

see. 266) This invention relates in general to improvements in radiation detector devices and in particular to a novel method of and means for extending the upper count rate of a radio activity detector tube. Also included in the present invention is a novel method of and means for sealing down the detector count output.

One serious defect present in the various radio activity detector tubes provided by the art such as the scintillation counter tube, the Geiger counter tube, and other similar devices is the limitation on the upper count rate with exposure to high intensity radio active fields. To explain, when a Geiger tube or the like is placed in a radiation field it will produce an output signal which increases with field intensity to a point where the tube becomes saturated, where beyond this, further increases in radiation intensity fail to produce a corresponding increase in the output response of the tube. This limitation is the result of the inherent dead time period of the tube following ionization and during which the tube is unresponsive. This period varies from tube to tube and in typical cases will range from something less than 100 microseconds for small insensitive tubes to several hundred (400 or 500) microseconds for the larger and more sensitive tubes. These periods of course determine the maximum counting rate capable of the tube. For example, a small insensitive tube whose dead time is as short as say 25 microseconds is capable of registering counts up to as high a .value as 40,000 per second, while a larger more sensitive tube whose dead time period maybe 500 microseconds is capable only of registering counts up to 2,000 per second.

In the past the so-called dead time" has been minimized by reducing the sensitive volume of the tube as far as practical, but this attack operates to reduce the sensitivity of the tube, thus rendering the same less effective to the detection of radiation in low intensity zones. Thus the present invention has for one of its objects the provision of a method of and a means for rendering a radio active detector tube operative in unusually high intensity zones of radiation and at the same time maintain the tubes sensitivity to the lower intensity zones of radiation.

It is another object of the present invention to provide a radio activity detector system which is operable over a wide range of field intensities. It is another object of this invention to provide a simple means for sealing down the count output of a radio activity detector tube.

It is another object of this invention to provide an extremely simple and inexpensive means of extending the operation of a, radio activity detector tube over an unprecedented range of field intensities.

These and other objects will become apparent upon the careful consideration of the following detailed description when taken together with the accompanying drawings, Fig. l of which is a simplified schematic diagram of one embodiment of the present invention.

Fig. 2 shows in schematic diagram form an.

alternate embodiment of the present invention; Figs. 3 and 4 show in schematic diagram the details of two practical embodiments of the present invention; and

Figs. 5 and 6 illustrate in graphical form certain characteristics of the present invention.

Very briefly, I have discovered that saturation (or jamming) level of a Geiger-Mueller counter tube or the like can be effectively and greatly extended by applying a normal direct current operating voltage across the tube whereby the same is rendered sensitive at all times and then momentarily raising the operating voltage across the tube, preferably .at periodic intervals. In the preferred embodiment the above action is accomplished by the use of voltage pulses superimposed upon the direct current operating voltage applied across the tube. These voltage pulses periodically increase the tube over-voltage (i. e., the voltage above counter threshold), and they have been found to effectively extend the upper sensitivity range of useful operation by several decades in field intensity.

In a second embodiment of the present invention the tube is normally held insensitive and is periodically driven into the counter region of the tube characteristic, preferably by a voltage pulse source. In this embodiment the tube is effectively turned on and off by the pulse source. The on period corresponds'to the duration of the pulse and theoif period corresponds to the duration of the period between pulses. This embodiment provides a scaling down feature in that it counts down the output signal directly at the tube itself. Accordingly this arrangement increases the saturation level of the tube and also permits, if desired, operation without the use of elaborate binary counter devices and the like between the tube and the customary ratemeter which is associated with the tube. With this type of operation the scaling factor, (that is the factor by which the counter output frequency is reduced) is equal to the reciprocal of the product of pulse duration ('Ip) times pulse recurrence rate (PRR). In equation form the scaling factor equals (T1 (PRR) In the discussion which follows the voltage signal for driving the detector tube into its overvoltage region in both the above embodiments is shown for purposes of simplicity; as being derived from a pulse generator. It is to be expressly understood however that the invention is not to be so limited in that the signal may assume various and diverse waveforms and need only act to periodically increase the over-voltage on the tube in the first embodiment or simply periodically drive the tube into its over-voltage operates to count the output from the detector l0 and register the same as an indication on a meter or other suitable device. Tube it is energized from or biased into the counter region of its characteristic by a suitable direct current source It the positive terminal of which is connected through a load resistor ii to the anode [2 of tube Ill, and the negative terminal of which is connected through resistor to the cathode H. Across resistor 25 is connected a suitable pulse generator IS for periodically increasing the over-voltage on the detector tube iii. In this embodiment direct current source to acting by itself operates to render detector tube Ill sensitive to detect radio activity. Pulse source l8 however superimposes in an aiding fashion a momentary pulse in series with source It on the detector tube Hi. This pulse as mentioned previously acts to drive the tube momentarily into its over-voltage region and if of sufiicient intensity effectively extends the saturation level of the tube. In general it has beeniound that the tubes saturation level increases with pulse amplitude up to a certain value after which there is no material improvement in the saturation level. Experiments indicate that for best results the amplitude of this pulse should be not less than approximately 20 percent of the value of the voltage source I6 and as will'be subsequently shown the duration of this pulse should preferably be for best operating results a small percentage of the dead time of the tube. Referring to Fig. 5 it will be shown how the duration of the pulse affects the upper sensitivity range of tube is. In this plot a tube having a normal dead time or 400 microseconds was employed. The pulse amplitude was set at 109 volts and the D. C. operating voltage l6 at 600, which value was above the threshold voltage for the tube. The abscissa is marked off in different values of pulse width expressed in terms or microseconds, and the ordinate in units of field intensity of. milli-roentgens per. hour. The .dotted line represents the normal saturation level above which the tube output count failed'to increase with further increases in field. intensity when the direct current voltage alone was applied. The pulse repetition rate was held constant at 2500 pulses per second and the pulse width varied. As will be noted as the pulse width is decreased from say microseconds to say approximately 2 microseconds the tube saturated at increasingly higher levels of radiation intensity. In general, the shorter the duration of the pulse the higher the upper saturating field level possible.

Referring to Fig. 6 there is shown in graphical form the characteristics of the same tube wherein the pulse duration was fixed at 2 microseconds and 100 volts in amplitude and the pulse repetition rate varied. From this figure it is apparent that the saturation level of the tube improves sharply with pulse repetition rate until it maximizes at a point where the repetition rate is substantially equal to the reciprocal of the dead time and then diminishes gradually thereafter. Thus it is apparent from both Figs. 5 and 6 that the upper count rate or the sensitivity of a Geiger-Muller tube or the like to higher levels of radiation can be greatly extended by pulsing the same in accordance with the teachings of the present invention and that an optimum condition is reached when the pulse repetition rate approaches or approximates the dead. time period of the tube and the pulse duration is made equal to a small percentage of the dead time period.

Referring now to Fig. 2 the second embodiment of my invention is shown. This embodiment differs from the embodiment shown in Fig. '1 only in that the direct current source I6 is omitted. In this case pulse generator 18 must have sufficient intensity to provide an energizing pulse for the counter tube iii. If pulses of smaller amplitude are desired then a voltage source similar to it can be inserted in series with pulse generator It and its magnitude made such that it is in sufficient by itself to energize the tube for counter operation. In this embodiment the output from detector tube is is scaled down in direct proportion to the reciprocal of the product of pulse duration (Tp) times pulse repetition period (PPR). Here the upper count rate or resolving time of the tube is extended by the scaling factor This embodiment permits, if desired, the use of a simplified count rate indicator M. In other words counting rate circuit Hi can be designed to handle a narrow range of input frequencies say between 0 and 25 and the input thereto kept within this range by properly adjusting the scaling factor provided by pulse generator H3 either by varying the pulse repetition rate or pulse duration or both. This feature of the invention obviates the use of elaborate scaling devices such as binary counters or the like between rate circuit M and the output of tube l8.

As will be recognized from both Figs. 1 and 2 the pulsing technique taught by the present in* vention may give rise to certain complications in that the exciting pulse provided by pulse generator l8 may itself tend to appear in the output of the counter tube and be registered by the count indicator 14. To eliminate this detect the circuits shown in Figs. 3 and 4 may be employed. Referring to Fig. 3 the energizingpulse source 158 is coupled through a conventional pulse trans? former is to the anode l2 of the counter tube.

shunting the secondary of pulse transformer 19 is a suitable potentiometer 2!, the movable tap of which is connected to the cathode of the tube I by way of an output load resistance I"! and battery 15. The other end of the secondary of pulse transformer I9 is connected to ground through a variable capacitance 22, which is adjusted to a value which is substantially equal to the inner electrode capacitance shown dotted at 23 of tube ill. In this fashion the phase opposed pulses appearing at the opposite ends of the transformer can be made to neutralize one another across the output load resistance l! by adjustment of tap 2| and capacitor 22 whereby only the counter output from tube It is made to appear in the output circuit.

An alternate arrangement of pulse neutralization is shown in Fig. 4. Here the pulse source output from the counter tube is coupled to the grid 24 of a first cathode follower type arrangement 25. The neutralizing pulse is coupled through the neutralizing capacitor 22 to the grid of an inverting tube 21, the plate of which is connected through resistance 28 to the cathode of cathode follower tube 25. In this circuit that component of the pulse coupled through stray capacity 23 is neutralized by that component of the pulse coupled through capacitor 22 to the inverter stage 21.

While only certain and specific embodiments of this invention have been shown and described therein it must be understood that I am fully aware of the many modifications possible thereof. Therefore this invention is not to be limited except insofar as dictated by the scope of the disclosure.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. The combination comprising a radio activity detector tube having anode and cathode electrodes, a source of voltage coupled across the electrodes of said tube having a magnitude such as to render said tube operative in the Geiger-Muller region of its characteristic, and means for periodically superimposing a momentary overvoltage pulse on said source, said overvoltage pulse being equal to substantially 20 per cent or more of said source of voltage.

2. In a radiation detector which includes a radio activity detector tube having a given dead time period and a supply voltage for said tube which is of a magnitude such as to render said tube normally operative in the Geiger-Muller region of its characteristic, the method for extending the sensitivity of said tube to include unusually high levels of radiation intensity which comprises the step of momentarily raising the voltage supplied to said tube by substantially 20 per cent or more above the normal supply voltage level for periods short in comparison with the dead time period of the tube and at intervals which approximate the dead time period of the tube.

3. In a radiation detector which includes a radio activity detector tube having a given dead time period and a supply voltage for said tube which is of a magnitude such as to render said tube normally operative in the Geiger-Muller region of its characteristic, a method for extending the sensitivity of said tube to include unusually high levels of radiation intensity which comprises the step of momentarily raising the voltage supplied to said tube by substantially 20 per cent or more above the normal supply voltage level for periods short in comparison with the dead time period of the tube.

4. In a radiation detector which includes a radio activity detector tube having a given dead time period, the method of extending the radiation detection sensitivity of said tube to include unusually high levels of radiation intensity which comprises, impressing a supply voltage across the electrodes of said tube of such a magnitude as to render said tube normally operative in the Geiger-Muller region of its characteristic, and then periodically raising the voltage supplied to said tube by substantially 20 per cent or more of the normal supply voltage level for periods short in comparison with the dead time period of the tube.

SAMUEL W. LICHTMAN.

References Cited in the file 01 this patent UNITED STATES PATENTS Number Name Date 2,443,857 Herzog June 22, 1948 2,506,435 Rossi et a1. May 2, 1950 

